Coordinate measuring machines are used for dimensional inspection of workpieces such as machine parts. A workpiece is secured to a fixed table, and a measuring probe is secured to a vertically movable ram which is also movable in a horizontal plane. To measure the position of a point on the workpiece, the probe is brought into contact with the point and the x, y and z measuring scales of the machine are read. To measure a distance between two points, the points are contacted successively, the coordinates of both points are read and distance is calculated from the coordinates. State-of-the-art coordinate measuring machines have refinements such as high resolution measuring systems, electrical contact probes, motor drives, computer controlled drives and computer acquisition and processing of data.
The accuracy of a coordinate measuring machine is limited by inaccuracies in the scales or other measuring systems and by faults in the guideways which establish orthogonality of machine motions. One approach to increasing accuracy is simply to improve the construction techniques and reduce tolerances of the system so that errors are reduced. However, the reduction of errors becomes progressively more expensive as required accuracies increase. Another approach is direct measurement of x, y and z errors at points throughout the machine working volume. This approach is impractical because of the huge amounts of data which must be stored for large machines and because of the time required to measure such data. A third approach is the measurements of errors in parametric form. That is, sets of error parameters are measured, for example, along three mutually orthorgonal axes and stored for future use. The x, y and z errors at any point in the measurement volume are calculated from the parametric errors. The calculated errors are then subtracted from the scale readings to determine actual workpiece coordinates.
The coordinate measuring machine has three sets of guideways which establish probe motion. Ideally, movement along each of these guideways should result only in linear motion and the scale reading would equal the linear displacement. In reality, however, there are scale errors and the guideways are not completely straight or perfectly free from twist. For a real machine, there are six degrees of freedom which produce errors during movement along each guideway. For each direction of movement, there are three linear errors, Dx, Dy and Dz and three rotational errors, Ax, Ay and Az. These six error parameters can be measured at a number of points along each direction of machine movement, resulting in an error matrix with 18 error parameters. From the matrix of 18 error parameters, the error at any point in the measurement volume can be calculated.
Various techniques have been used for the measurement of parametric errors. Laser interferometer techniques are well-known for measuring displacement errors with high accuracy. Dual frequency interferometer techniques have also been utilized for measurement of straightness and roll as disclosed in U.S. Pat. No. 3,790,284, issued Feb. 5, 1974 to Baldwin. A system utilizing partitioned photocells to detect pitch and yaw of a stage is disclosed in U.S. Pat. No. 3,715,599 issued Feb. 6, 1973 to Marcy. A four quadrant angular movement sensor is disclosed in U.S. Pat. No. 3,765,772 issued Oct. 16, 1973 to Willett. One prior art approach to measurement of parametric errors utilizes a Hewlett-Packard 5526A laser measuring system, which is described in "Calibration of a Machine Tool," Hewlett-Packard Laser Measurement System Application Note 156-4. The system is transportable between machines but the machine calibration time is about 40 hours. In addition, a different setup is needed for each measurement and setup errors are difficult to avoid. A system for measuring the six error parameters along each axis of motion of a measuring machine is disclosed in U.S. Pat. No. 4,261,107, issued Apr. 14, 1981 to Coleman et al. The system utilizes interferometric techniques for measuring each of the error parameters and, thus, requires a dual frequency laser to measure displacements perpendicular to the laser beam axis. As a result, the system is complex and expensive. Furthermore, different fixed measurement arrangements are utilized for each of the three axes of motion of the machine, thereby further adding to the complexity and cost of the system.
Since the error parameters associated with a particular machine remain relatively fixed with time, it is desirable to provide a method and apparatus for machine calibration wherein a calibration device can be attached to the machine, the calibration procedure can be performed in a short time and the calibration device can then be removed for use with another machine. Such a calibration system must provide high accuracy error measurement and must be easily attached to the machine and easily detached after use. The calibration procedure can be repeated as necessary through the life of the machine.
It is a general object of the present invention to provide improved coordinate measuring machines.
It is another object of the present invention to provide methods and apparatus for calibrating the position of a movable element relative to a fixed element in a machine.
It is yet another object of the present invention to provide methods and apparatus for improving the accuracy of coordinate measuring machines.
It is still another object of the present invention to provide methods and apparatus for measuring the parametric errors associated with a movable element relative to a fixed element.
It is another object of the present invention to provide a calibration system for a coordinate measuring machine which is easily attachable and detachable from the machine.
It is yet another object of the present invention to provide methods and apparatus for calibrating a coordinate measuring machine.
It is a further object of the present invention to provide methods and apparatus for accurately measuring parametric errors along each of three directions of movement in a coordinate measuring machine.